An article in BJOG (a journal of Obstetrics and Gynaecology) states that some parents are misinformed by doctors about the zygosity of their twins. According to the article 14.7% of parents were misinformed about whether their twins are identical or fraternal.
Zygosity is whether the twins came from one egg or two.
Mono-zygotic = identical twins — mono meaning one zygote meaning fertilized egg
Di-zygotic = fraternal twins — di meaning two
There seems to be a fairly common misperception amongst doctors and midwives that if there are two placenta the twins are di-zygotic. This is not true. In fact, 25-30% of monozygotic twins have two placentas. And a high percentage of diamniotic twins have fuzed placentas which can look like one placenta.
You see, it all depends on when the zygote splits. If it splits early, around two days, there will be two placentas, two chorions (outer sac) and two amnions (inner sac). If it splits a little later there will be one placenta, one chorion and two amnions – called monochorionic/di-amniotic. Another few days and you have one of each – called mono-chorionic/mono-amniotic. Still later you get conjoined twins.
How does one determine zygosity?
The best time to find out is during pregnancy. Zygosity is VERY important in terms of pre-natal care. Unfortunately there is only a small window of time during which an ultrasound will definitively show the line between amniotic sacs in a mono-chorionic twin pregnancy. The window is 12-18 weeks. Often before the woman knows she is having twins.
Monochorionic monoamniotic twins are the most at risk. Their cords can become entangled very easily which often leads to the death of both twins. Live birth rates are low.
Monochorionic Diamniotic twins need to be monitored for Twin to Twin Transfusion, TTTS. I am very thankful that my OB sent me to Toronto to see Dr. Jon Barrett to get his opinion on pre-natal care. I had an ultrasound there at 16 weeks and they could tell that my twins were monochorionic dianmiotic so we were at risk for TTTS. So I was monitored with bi-weekly ultrasound for until 28 weeks and then weekly ultrasounds. Not regular ultrasounds but half hour long ones that measured growth, amniotic fluid levels and blood flow/pressure in the umbilical cords.
Many parents want to know the zygosity of their twins for a variety of reasons. Health is an good reason to know the zygosity of your twins. Knowing the zygosity may be helpful if there are genetic illnesses.
Twin relationships are also often different with identical and fraternal twins. All twin relationships are special but identical twins seem to have an extra special connection.
I personally know of several families who have twins who look suspiciously identical yet they were told they are fraternal. Hmmmm.
So if you are pregnant with twins, or know someone who is, pass this along so they can get an ultrasound early enough to find out. Also, contact your local multiple births chapter and check out Multiple Births Canada (MBC), their fact sheets have all the information you need summarized into easy reading packages.
If your twins are already born the only definitive way to find out is a DNA test.
Here is the MBC fact sheet on zygosity. It has information on getting DNA testing at a reasonable price by Dr. Fiona Bamworh.
There’s also Easy DNA.http://www.easydna.ca/
Great post Erin! I was fortunate in that I had my ultra sound at 13 weeks and knew right away that my babies were Monochorionic Diamniotic I was sent to the local hospital’s High Risk OB clinic but at 18 weeks was sent to Mount Sinai to Dr. Ryan when it was discovered that my little guys had TTTS – the TTTS didn’t develope to the state that required surgery but they did have IGUR which was really bad and I was told to be prepared because they were pretty sure Tanner wouldn’t make it. I spent every week going back and forth to MS hospital and never knew what to expect it was horrible. Thankfully they overcame so many obstacles and just look at them today! It is very important that women know if their babes are fraternal or not because it makes a huge difference to the pre-natal care they must get and unfortunately the general population has no idea of the risks involved when one is pregnant with twins. I know I didn’t at first.